1. Field of the Invention
The present invention relates to an A-D converter circuit which converts analog signals into digital signals.
2. Description of the Prior Art
An A-D converter circuit is used to convert analog signals into digital signals and, conventionally, there are present various kinds of circuit configurations for such A-D converter circuits. Also, there exist various methods in which image informatio is A-D converted, the A-D converted image information is stored into a display memory, and the stored digital data is modified so as to create an image. In this case, from the viewpoint of versatility, a low-cost A-D converter circuit is required.
When used to load the image information, from the viewpoint of actual requests for resolution and the like, an A-D converter circuit capable of outputting 5 bits is desirable. However, the A-D converter circuit capable of outputting 5 bits presents a problem that it is much more expensive than an A-D converter circuit capable of outputting 4 bits. The reason why the circuit of 5-bits output is desired is as follows:
In FIG. 4, there is shown a circuit diagram of a prior art A-D converter circuit which is adapted to output 2 bits. This 2-bit output A-D converter circuit 10 comprises 3 analog comparison circuits ACP 1.about.ACP 3 and 4 division resistances R 1.about.R 4. The outputs of the analog comparison circuits ACP 1.about.ACP 3 are coded by an encoder 11, and the coded data are latched by a latch 11a. FIG. 5 illustrates a table which shows the relationship between the input and output of the above-mentioned encoder 11.
Also, the A-D converter circuit capable of outputting 4 bits comprises 15 analog comparison circuits and 16 division resistances, while the A-D converter circuit of 5-bits output requires 31 analog comparison circuits and 32 division resistances. Thus, in the 5-bit outputting A-D converter circuit, the number of the division resistances and analog comparison circuits is substantially increased, and at the same time the division resistances require a very high accuracy and the analog comparison circuits require a very high sensitivity. When the sensitivity is enhanced, noise must not be allowed to get mixed in or occur, requiring an effective noise-preventive countermeasure. Also, since the encoder has 31 inputs, the number of gates of the encoder are increased accordingly. For these reasons, the 5-bit output A-D converter circuit becomes very expensive.
On the other hand, in a computer system, a display control system displaying on a CRT or the like is arranged such that, after the display data to be displayed is once written into and stored in a display memory, it reads out the data from the display memory and displays it on the CRT. In this case, in order to enhance a pixel density or realize color display, it is necessary to increase the capacity of the above-mentioned display memory. In the color display, the more kinds of colors (color gradations) are used, the more the capacity of the display memory must be increased. Also, in case of a monochrome display, as the number of display gradations is increased, the capacity of the display memory is increased, which is a problem to be solved.